Rooms are already being converted at VWL but I doubt they will begin accepting reservations for parties of 5 until after all of the renovations are complete.
Grand Floridian is the only current resort with Studios that sleep 5.
We just stayed in as a party of 5 in a VGF villa, and found the size to be similar to what we're used to in a deluxe hotel room on property with a daybed.
The daybed in the villas is similarly-sized to the daybed in a deluxe room, and it will definitely sleep an adult. I am 5'10" and had no trouble fitting on it, and my almost-13-year-old daughter (who slept on it) loved the bed.